Steel body having alloyed zinc coating and method of producing such coating



June 6, 1961 P. E. SCHNEDLER 2,986,808

STEEL BODY HAVING ALLOYED ZINC COATING AND METHOD OF PRODUCING SUCHCOATING Filed Aug. 4, 1958 INVENTOR. Fin/L E. Scw/VEULE United S atesPatent F STEEL BODY HAVING ALLOYED ZINC COATING AND METHOD OF PRODUCINGSUCH COATING Paul E. Schnedler, Middletown, Ohio, assignor to ArmcoSteel Corporation, Middletown, Ohio, a corporation of Ohio Filed Aug; 4,1958, Ser. No. 753,015 8 Claims. (Cl. 29-1962) This invention relates toa steel body having an alloyed zinc coating and to a method of producingsuch a coating. Generally speaking, when a steel body is coated withzinc by the hot dip method, it has hen considered desirable to conductthe operation in such a manner that alloying of the coating wasminimized and preferably substantially avoided. For certain particularpurposes, however, it has been desired to produce a zinc coating whereinthe zinc was alloyed with iron. The production of an alloyed zinccoating has been very difficult to accomplish in a homogenous and evenmanner. Photomicrograph studies of cross-sections of alloyed coatingshave shown that the coatings not only are non-uniform in the degree ofalloying from one area to another, but that they are not homogenous fromthe base metal out to the surface and often consist of a plurality ofphases wherein different proportions of the alloying metals are found indifferent strata. The principal difficulty however has been that wherealloying of the coating has proceeded to the surface, the alloyingaction has tended to accelerate while in areas where the alloying wasslower in starting, the alloying has proceeded less rapidly so that avery uneven and blotchy appearance resulted.

With the foregoing considerations in mind, it is an object of thepresent invention to provide a steel body having essentially a one-phaseiron zinc alloy coating wherein the alloy is homogeneous substantiallyfrom the interface between the alloy and the iron base metal out to thesurface of the coating. It is another object of the invention to providesuch a body having a uniform alloy coating over its entire area wherebythe appearance of the coated body is uniform and smooth.

It is still another object of the invention to provide a body having analloyed zinc essentially one-phase coating containing-from about eightpercent to about twelve percent iron whereby to produce a coating whichis ductile and will not chip, peel, or flake in a forming or drawingoperation and which will provide an excellent paintable surface. Itisstill another object of the invention to provide a method of treatingmetal bodies and particularly steel strip to produce thereon a coatingof the character described herein and further to provide such a methodwhereby the alloying step can be carried on at the normal speed at whichthe strip passes through the zinc coating apparatus.

These and other objects of the invention which will be pointed out inmore detail hereinafter or which will be apparent to one skilled in theart upon reading these specifications are accomplished by that methodand in that form of which the following is an exemplary embodiment.

Reference is made to the drawing forming a part hereof and inwhich:

FIGURE. 1 is a semi-diagrammatic cross-sectional view of a typical zinccoating pot showing an apparatus for practicing the method and producingthe article herein described.-

2,986,808 Patented June 6, 1961 have stemmed primarily from the factthat an unalloyed zinc coating is highly reflective to heat while analloyed coating has a very low reflectivity. To state the aboveproposition in' the converse, a black body has an absorptivity oremissivity of one hundred percent. In other words, a black body willabsorb one hundred percent of the heat impinging upon it or if heatedinternally will emit one hundred percent of the heat. The absorptivityof an alloyed zinc iron coating having a rather dull mat finish isapproximately seventy-eight percent. The absorptivity of a moltenunalloyed zinc coating is approximately 8.5 percent. 7

When an unalloyed zinc coating is placed upon a steel strip it requiresheat to carry out the alloying step. In the past, this heat has alwaysbeen externally applied as by passing the strip through a furnace or bycausing a direct flame to impinge upon the coating. When heat is appliedin this manner to an unalloyed zinc coating, as soon as alloying takesplace in a given area, the absorptivity of the coating increasesdramatically. The result of this is that more heat is absorbed by thoseareas where alloying has been effected and therefore the alloying actionspeeds up, and if allowed to continue, develops a multi-phase structure.Conversely, in those areas where alloying has not begun to take place,the high reflectivity or low absorptivity of the surface minimizes theheat supplied so that alloying action in these areas is retarded. Itmight be said that non-uniformity of the coating is multipliedsubstantially nine fold.

Briefly, in the practice of the present invention, the heating of thestrip necessary to produce the alloying action is applied from within.This may be accomplished by heating the strip by electric resistance,or' by induction heating, or a combination of induction and resistanceheating. In the claims the terms electrically heating, electricalheating and heating electrically are used to specify a heating fromwithin either by electrical resistance or by a combination of inductionor resistance heating. It is a well understood fact that when an objectis heated by induction, ferromagnetic materials will heat much morerapidly and readily than non-ferrous materials. And, it is known thatinduced currents tend to flow on the surface of ferromagnetic materials.In this instance, with a sandwich type construction where a non-ferrousmetal encloses a ferrous metal, the induction heating will cause a heatconcentration at the non-ferrous-ferrous interface causing alloyingbetween the two metals. When this well known concept is applied to theproblem at hand, the highly divergent absorptivities of alloyed zinc andunalloyed zinc, instead of compounding the non-uniformity of thecoating, actually operate in favor of a uniform coating. Thus, when thecomposite'body is heated from within as by induction heating, as soon asalloying proceeds to the surface in a given area the reflectivitydecreases and the emissivity increases so that the heat supplied to thatarea is emitted from that area to the ambient atmosphere more rapidly,while the heat applied to an unalloyed portion where the reflectivity ishigh and the emissivity is only about 8.5 percent, is retained sincesuch portions are in a manner of speaking insulated; and with more. heatapplied the alloying proceeds at a greater rate. The general effect isthat the entire coating is uniformly alloyed and a non-uniform ormottled appearance is avoided.

Furthermore, where induction heating is used as above outlined, heatingcan be much more rapid and the heat input can be controlled veryaccurately to maintain the issues from the coating pot at a temperatureof about 860 F. The maximum rate of alloying of zinc and iron o'ccurs'attemperatures of 900 to 950 F. By means of an induction coil, thetemperature or the strip must be raised from about 860 F. to between 900and 950 F. and held there for a suificient time for iron molecules tomigrate out to the surface of the coating. By induction heating this canbe accomplished in a matter of a few feet of travel of the strip andover a period of a few seconds. An advantage of this method is that itcan be applied to existing zinc coating lines where space limitationsare a factor. Because of the rapidity with which the body may be heated,the alloying operation may be carried out at the speed of the zinccoating apparatus.

Generally, it will be advantageous to accomplish the heating immediatelyabove the coating pot, so that the alloying will be initiated before themolten coating begins to freeze in the conventional spangled pattern.Spangled coatings can of course be reheated and alloyed, but theoutlines of the spangles will tend to persist and the oxide impuritiesat the spangle boundaries will be retained in the alloyed coating.

In the drawing there is shown a conventional zinc pct containing themolten zinc 11. The strip 12 enters the pot from a source of supply (notshown) after having been given a desired pretreatment and after passing.12 passes or as shown in FIGURE 3 it may comprise two flat loops 15band 150 between which the strip 12 passes.

If desired,'it' is of course possible to provide additional loops 17 asshown in FIGURE 2, or additional loops 18, 19, as shown in FIGURE 3, asnecessitated by the gauge and speed of the strip, so as to produce therequired heating within the available time.

It will be within the skill of the electrical engineer to determine thephysical dimensions of the loops and the power to be applied as well asthe frequency in order to achieve the required degree of heat within therequired space of time. Experimentally when using zinc coated strippreviously given an unalloyed zinc coating and now at room temperature,perfect alloying of the zinc coating was achieved with a single turncoil as shown in FIGURE 2, supplied with ten kilornatts power at afrequency of 185 kilocycles. It must be borne 'in mind that in thisexperiment the inductive heating apparatus is required to raise thetemperature of the strip from room temperature to about 950 F. Ofcourse, where the alloying operation 'follows immediately the coatingoperation, the temperaspecification and claims, indicates that althoughundoubtedly one or more phases are present at the interface between thealloy and the iron base metal, these are practical-lyimpossible toidentify, and that to all intents and purposes, the alloy coating is aone-phase alloy. The surface of this coating has an excellent paintablesurface and appears to have good corrosion resistance. sired, aphosphate coating may be applied to the alloyed surface but this is notconsidered to be necessary to give it paintable qualities. The coatingis very ductile and does not flake, crack or chip during a formingoperation.

Where a zinccoating bath containing a small percent-' v age ofalurninumis used, the alloy coating tends to fall described above, perfectessentially If de-" 4 a more readily within the range of 8 percent to 12percent iron, although this range can easily be achieved withoutaluminum if overheating is avoided. It has been known for many years (asfor instance as disclosed in US. Patent 2,197,622) that the addition ofaluminum in small percentages to molten zinc coating baths willfacilitate the production of galvanized sheet and strip having s11-perior adherence of the coating. In this instance the coating will becomposed primarily of free zinc with very little alloy formation at theinterface between the base metal and the coating. Now it has beendetermined that if this type of zinc coated steel strip (which includesabout .001-.35% aluminum in the coating) is heated, before the zincfreezes on the strip, until the coating is completely alloyed, thecoating will be smooth and non-spangled and have particularly desirableproperties as regards ductility, adherence, and paintability; The ironcontent of this zinc coating which contains a small amount of aluminumwill be on the order of 812 percent. Excessive heating will causeadditional diffusion of iron into the alloyed zinc coating layer, whichtends to destroy these desirable properties. These high iron contentalloys may reach 20-35% iron.

It is to be understood that while the above mentioned examples haveincluded references to induction heating, resistance heating means orother internal heating means are equally well adapted to supply thenecessary heat, as long as these heating means afford a heatconcentration to the ferrous metal core or the interface, rather than tothe non-ferrous surface layer. There-are certain advantages to eachheating method. It is possible to add the induction heating equipment toexisting coating facilities without alteration of existing equipment.Resistance heating means are sometimes considered to be more economical,but adaptions to existing equipment must be made so that power sourcescan contact the strip, preferably in the coating bath and at a secondpoint above the coating bath after the coating metal has solidified andthe alloying is completed.

It will be clear that various modifications may be made withoutdeparting from the spirit of the invention and that details describedherein have been by way of example only.

What is claimed is:

l. A steel body having essentially a one-phase coating of asubstantially iron-zinc alloy containing from about .001% to about 35%of aluminum, said alloy coating being ductile and having goodpaintability and produced by coating said body with zinc containing saidpercentage of aluminum, and subjecting said coated body to elecricalheating from within and maintaining said body in said heated conditionfor a time sufiicient to permit iron molecules to migrate to the outersurface of the coating.

2. A steel strip having essentially a one-phase coating of asubstantially iron-Zinc alloy containing from about .001% to about 35%of aluminum, said alloy coating being ductile and having goodpaintability and produced by coating said strip with zinc containingsaid percentage of aluminum, and subjecting said coated strip toelectrical heating from within and maintaining said strip in said heatedcondition for a time suflicient to permit iron molecules to migrate tothe outer surface of the coating.

. produced by coating said strip with zinc containing said percentage ofaluminum, and subjecting said coated strip to electrical heating fromwithin and maintaining said strip in said heated condition for a timesuflicient to permit iron molecules to migrate to theouter surface ofthe coating.

4. The method of producing on a steel body an alloyed coating of zinccontaining'from about .001% to about aeeaeoe 35% of aluminum, whichincludes the steps of electrically heating said coated body to atemperature of from about 900 F. to about 950 'F., and maintaining saidbody at said temperature for a time suflicient to permit iron moleculesto migrate to the outer surface of the coating.

5. The method of producing on a steel strip essentially a one-phasealloyed coating of zinc containing from about .001,% to about 35% ofaluminum, which includes the steps of electrically heating the zinccoated strip, from within, to a temperature of from about 900 F. toabout 950 F., and maintaining said strip at said temperature for a timesufiicient to permit iron molecules to migrate to the outer surface ofthe coating.

6. The method of producing on a steel strip essentially a one-phasealloyed coating of zinc containing from about .001% to about .35 ofaluminum, which includes the steps of heating the strip, after itemerges from the coating pot, by electrical heating, to a temperature offrom about 900 F. to about 950 F., and maintaining said strip at saidtemperature for a time sulficient to permit iron molecules to migrate tothe outer surface of the coating.

7. The method of producing on a steel strip an alloyed coating of zinccontaining from about .00l% to about .35 of aluminum, which includes thesteps of electrically inducing in said stn'p, after it emerges from thecoating pot, an increase in temperature to from about 900 F. to about950 F., and maintaining said strip at said temperature for a timesufficient to permit iron molecules to migrate to the outer surface ofthe coating.

8. The method of producing a non-spangled, alloyed coating of zinccontaining from about .001% to about .35 of aluminum ona ferrous metalstrip, which includes the steps of applying to said strip saidzinc-aluminum alloy coating internally heating the coated stripelectrically, while the coating metal is still molten, to a temperatureof from about 900 F. to about 950 F., and maintaining said strip at saidtemperature for a time suflicient to diffuse iron from the base metalinto the coating whereby to develop a homongeneous, substantially singlephase coating of iron-aluminum-zinc alloy of an iron content between 8and 12%, said alloy being ductile and having good paintability.

References Cited in the file of this patent UNITED STATES PATENTS1,307,853 Dimm June 24, 1919 1,430,648 Herman Oct. 3, 1922 1,890,463Herman Dec. 13, 1932 1,998,496 'Fiedler Apr. 23, 1935 2,172,933 Daesemet al. Sept. 12, 1939 2,197,622 Sendzimir Apr. 16, 1940 2,664,873 GrahamJan. 5, 1954 2,703,766 Ellis et a1 Mar. 8, 1955 2,744,032 Highfield May1, 1956 FOREIGN PATENTS 586,376 Great Britain Mar. 17, 1947 781,594Great Britain Aug. 21, 1957 OTHER REFERENCES Burns and Bradley:Protective Coatings for Metals," 2nd edition, Reinhold Pub. C0rp., 1955,pages 108-111.

1. A STEEL BODY HAVING ESSENTIALLY A ONE-PHASE COATING OF A SUBSTANTIALLY IRON-ZINC ALLOY CONTAINING FROM ABOUT .001% TO ABOUT .35% OF ALUMINUM, SAID ALLOY COATING BEING DUCTILE AND HAVING GOOD PAINTABILITY AND PRODUCED BY COATING SAID BODY WITH ZINC CONTAINING SAID PERCENTAGE OF ALUMINUM, AND SUBJECTING SAID COATED BODY TO ELECTRI- 